Switching mixers with rejection properties on one or more odd higher harmonics are well known, see, e.g., J. A. Weldon et al., “A 1.75-GHz highly integrated narrow-band CMOS transmitter with harmonic-rejection mixers”, IEEE Journal of Solid-State Circuits, Vol. 36, No. 12, December 2001, pp 2003-2015. A harmonic-rejection (HR) mixer allows simplifying the RF filtering. A typical HR mixer known in this field is an active (Gilbert) type of mixer. However, passive mixers are known to provide a better 1/f noise and higher linearity. On the other hand, a passive HR mixer is more difficult to implement.
HR mixers are also disclosed in non pre-published European patent application 072 90 983.1, filed Aug. 7, 2007 for “Harmonic Rejection Mixer Unit and Method for Performing a Harmonic Rejection Mixing” Ser. No. 12/671,734 , and having co-inventors in common with the current invention.
FIG. 1 in the drawing appended to the current document is copied from EP 072 90 983.1 and is a block diagram of a HR mixer 100. Mixer 100 is a quadrature mixer. Mixer 100 comprises an RF (radio/frequency) input 102 and an IF (intermediate frequency) output 104. Output 104 provides the in-phase output signal “I” and a quadrature output signal “Q” whose phase is shifted over 90° relative to signal “I”. Mixer 100 comprises amplifiers 106 and 108, whose differential inputs are connected to input 102 via a switching network and whose differential outputs provide signals “I” and “Q”. The switching network at input 102 is made up of switching FETs 110, 112, 114, 116; switching FETs 118, 120, 122, 124; switching FETs 126, 128, 130, 132; switching FETs 134, 136, 138, 140; and resistors 142, 144, 146, 148, 150 and 152. FETs 110 and 114 have their main current paths connected to input 102 via resistor 142. FETs 112 and 116 have their main current paths connected to input 102 via resistor 144. FETs 118, 122, 126 and 130 have their main current paths connected to input 102 via resistor 146. FETs 120, 124, 128 and 132 have their main current paths connected to input 102 via resistor 148. FETs 134 and 138 have their main current paths connected to input 102 via resistor 150. FETs 136 and 140 have their main current paths connected to input 102 via resistor 152. The input network together with resistors 154, 156, 158 and 160, configure amplifiers 106 and 108 as summing amplifiers.
FIG. 2 is a diagram 200 of the control signals used to switch FETs 110-140. FETs 126 and 128 are controlled by a signal GS10. FETs 130 and 132 are controlled by a signal GS11. FETs 118 and 120 are controlled by a signal GS3. FETs 122 and 124 are controlled by a signal GS4. FETs 110 and 112 are controlled by a signal GS8. FETs 114 and 116 are controlled by a signal GS9. FETs 134 and 136 are controlled by a signal GS6. FETs 138 and 140 are controlled by a signal GS7. Signals 202-216 are derived from a local oscillator (not shown). As a result, signals “I” and “Q” form the weighted sum of the switched input signals, effectively forming the result of mixing the input signal at input 102 with control signals generated by the local oscillator.
FIG. 3 is a diagram 300 of the effective resulting mixing waveforms for the “I” and “Q” signals. The mixing waveforms can be thought of as being build up by selectively combining the control signals of FIG. 2 so as to approximate the sine shape.
Although circuit 100 works well, the rejection of the third and fifth harmonic of the oscillator signal turns out to be hampered by the finite, frequency-dependent input impedance of amplifiers 106 and 108. This input impedance is a factor in the weighting of the RF currents used in creating the pseudo-sinusoidal mixing waveforms of FIG. 3. For example, in order to obtain a harmonic rejection of 65 dB the magnitude of the input impedance should be less than 0.3% of the RF series resistance, used to convert voltage into a current. In a typical case, the RF series resistance has a value of 100 Ohm. The input impedance should then have a value lower than 0.3 Ohm. This can be difficult to realize, particularly as it needs to be maintained over the frequency range of the received RF signal and its harmonics. The input impedance results from feedback applied to a bandwidth-limited amplifier, and the nature of the impedance typically changes with frequency from inductive to capacitive, peaking to a real value somewhere in between.
This dependence on the frequency makes it very difficult to compensate for the input impedance, for example in the values of the resistors used in circuit 100.
Despite their better 1/f noise and better linearity, passive switching mixers (such as the mixer 100) with harmonic-rejection properties are currently not widely used. One reason for this, is that the input impedance of the IF amplifier cannot be made sufficiently small to provide the required accuracy in the weighting of RF currents that is needed to achieve high values of harmonic-rejection. The invention proposes an improved topology for such a passive HR mixer.